Method for the production of air gases by the cryogenic separation of air with improved front end purification and air compression

ABSTRACT

A method and apparatus for the production of air gases by the cryogenic separation of air with front end purification and air compression can include using an available compressed dry gas such as nitrogen, oxygen, stored purified air, or synthetic air to repressurize the adsorber without diverting any of the purified air just exiting the currently on-line adsorber or changing the flow rate of the main air compressor or air sent to the cold box. This enables the main air compressor (MAC) to operate at a relatively constant flow rate while also sending a relatively constant air flow to the cold box during this repressurization step, thereby reducing the risks of process upsets and minimizing capital expenditures related to the MAC and other warm-end equipments.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/468,926, filed Mar. 24, 2017, which is incorporated by referenceherein in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention generally relates to a method for producing airgases by the cryogenic separation of air, and more particularly a methodfor improving the front end purification and air compression associatedwith said cryogenic separation.

BACKGROUND OF THE INVENTION

Air separation plants separate atmospheric air into its primaryconstituents: nitrogen and oxygen, and occasionally argon, xenon andkrypton. These gases are sometimes referred to as air gases.

A typical cryogenic air separation process can include the followingsteps: (1) filtering the air in order to remove particulates that mightdamage the main air compressor (MAC); (2) compressing the pre-filteredair in the main air compressor and using interstage cooling to condensesome of the water out of the compressed air; (3) passing the compressedair stream through a front-end-purification unit to remove residualwater and carbon dioxide; (4) cooling the purified air in a heatexchanger by indirect heat exchange against process streams from thecryogenic distillation column; (5) expanding at least a portion of thecold air to provide refrigeration for the system; (6) introducing thecold air into the distillation column for rectification therein; (7)collecting nitrogen from the top of the column (typically as a gas) andcollecting oxygen from the bottom of the column as a liquid.

With respect to step (3), the impurities are removed upstream the heatexchanger because, in the absence of such a pretreatment of the air,these impurities, particularly CO₂ and/or water vapor, would inevitablycondense and solidify as ice while the air is being cooled to acryogenic temperature, something which would cause problems of blockingin the cryogenic separation equipment or unit, especially the heatexchangers, distillation columns, etc., and thereby lead tomalfunctioning of or damage to the equipment or unit.

To avoid these problems, it is common practice to pre-treat the air thathas to be cryogenically separated before this cryogenic separation. Thispretreatment of the air is usually called “front end” scrubbing orpurification, since it is carried out upstream of the cryogenicseparation unit. Currently, the air is pretreated by a TSA (TemperatureSwing Adsorption) process or by a PSA (Pressure Swing Adsorption)process depending on the case.

Conventionally, a TSA process cycle comprises the following steps:

-   -   a) purification of the air by adsorption of the impurities at        superatmospheric pressure;    -   b) depressurization of the adsorber down to atmospheric        pressure;    -   c) regeneration of the adsorbent at atmospheric pressure via        introduction of a hot gas, for example the residual gases or        waste gases, typically impure nitrogen coming from an air        separation unit (ASU) cold box and heated by means of one or        more regeneration heaters;    -   d) cooling of the adsorbent, especially by continuing to        introduce into it the same residual gas stream coming from ASU        cold box , but not heated;    -   e) repressurization of the adsorber using the purified air,        coming, for example, from another adsorber which is in        production phase.

Moreover, as regards a PSA process cycle, this includes substantiallythe same steps a), b) and e), but is distinguished from a TSA process bythe residual gas or gases not being heated during the regeneration step(step c)), and therefore by the absence of step d) and, in general, ashorter cycle time than in a TSA process.

In modern designs, the front-end purification units operate based onadsorption and desorption of the impurities with varying pressures(pressure swing adsorption—“PSA”) or temperature (temperature swingadsorption—“TSA”). In either case, there are typically at least twoseparate vessels configured in parallel and operating in a permutablefashion. This means that as one bed is online and adsorbing impuritiesuntil it reaches its capacity, the other bed is being regenerated (i.e.,undergoing cleaning to remove the impurities). Once the first bedreaches its capacity for impurities, the configuration is switched,thereby causing the second bed to come online and start its adsorptionphase, while the first bed (which was originally in adsorption phase)begins its regeneration phase. Such TSA or PSA air purificationprocesses are described, for instance, in documents U.S. Pat. Nos.3,738,084; 5,531,808; 5,587,003; and 4,233,038; all of which are hereinincorporated by reference in their entireties.

Regeneration typically includes (1) depressurizing to near atmosphericpressure in order to remove the trapped gases and vent them to theatmosphere; (2) cleaning of the adsorbent bed by introduction of a dry,clean gas (often times waste nitrogen gas produced by ASU cold box)initially at an elevated temperature and then at an ambient temperature;and (3) repressurizing to feed pressure using clean product gas (i.e.,purified air) from the other online bed.

FIGS. 1 and 2 represent processes known heretofore. FIG. 1 represents aflow diagram of front end purification unit 10, in which first adsorberA is in its adsorption cycle and second adsorber B is in the cleaningstage of its regeneration cycle. In this setup, valves 3A, 4A, 5B and 6Bare open, while valves 3B, 4B, 5A, 6A, and 7 are closed.

In the example shown, 100 moles of compressed wet air 2 enters front endpurification unit 10 after being compressed in the main air compressor(MAC) 1. Compressed wet air 2 passes through open valve 3A and intofirst adsorber A, wherein substantially all of the water vapor andcarbon dioxide are captured thereby forming 100 moles of purified dryair 12. Purified dry air 12 then passes through valve 4A, leaving frontend purification unit 10 and then on to the cold box 20 forrectification into an oxygen-enriched stream and a nitrogen-enrichedstream.

Regeneration gas 14, which is preferably taken as a slip stream of thenitrogen-enriched stream, is introduced into front end purification unit10 and passes through open valve 5B before entering second adsorber B,wherein regeneration gas 14 removes the remaining water and carbondioxide to form wet waste gas 16, which is withdrawn and passes throughopen valve 6B before being vented to the atmosphere. Optionally,regeneration gas 14 can be heated by a heater (regeneration heater)prior to entering adsorber B in order to improve the regeneration of theadsorber vessel. If heating is used, then the adsorber is typicallycooled down using the same regeneration gas 14, just without theoptional heating.

FIG. 2 represents a flow diagram of front end purification unit 10 inwhich first adsorber A is still in an adsorption cycle and secondadsorber B is in a pressurization phase (i.e., pressurizing back tonormal adsorption pressure to be ready for its adsorption phase). Inorder to accomplish this, valves 5B and 6B are switched from open toclosed, which halts the flow of regeneration gas 14 to second adsorberB. Valve 7 is opened, thereby allowing a small portion of the purifiedair flow to divert to second adsorber B. In the example shown, the MAC 1has increased its output to 105 moles (from 100 moles) in order toaccommodate the 5 moles of air being used to pressurize second adsorberB.

Once second adsorber B is at the desired pressure (e.g., same pressureas adsorber A), valves 3B and 4B would be switched to open, and thenvalves 3A and 4A, and 7 would be switched to the closed position. Oncevalve 7 is closed, the MAC 1 output decreases back to 100 moles in orderto maintain a constant flow of air to the cold box 20. Valves 5B and 6Bwould remain closed. This switches the setup such that second adsorber Bwould be in its adsorption cycle while first adsorber A would bedepressurized down to atmospheric pressure via a depressurizing valve(not shown) and subsequently regenerated by switching valves 5A and 6Ato open.

During the pressurization phase, typically between 4 to 5%, of theprocessed air from the adsorber in production is used to re-pressurizethe other adsorber prior to being put online. This is known as switchlosses. The losses are experienced either by less air flow going to thecold box (while keeping MAC flow constant) or by ramping the MAC flowrate in order to keep a constant flow rate to the cold box and thusminimize cold box disturbances. However, both of these setups have majordrawbacks.

If the MAC flow rate is kept constant, then periods of repressurizationresult in a change in air flow to the cold box, which leads to adisturbance resulting in a process upset, which can make controlling theoverall process more difficult, as well as momentarily reducing productoutputs. In instances in which flow to the cold box is kept constant (asshown in FIG. 2), ramping the flow rate of the MAC requires a MAC thatis oversized for the majority of its operation. As such, a larger thannecessary MAC, as well as warm-end equipments, such as air pre-coolingsystem and front end purification unit 10 are typically purchased, whichleads to higher capital expenditures.

U.S. Pub. 2014/0013798 attempts to solve this problem by using variableair flows through turbine 27 (typically called a lost air turbine) toallow for a constant air flow to the distillation column. Duringrepressurization cycles, the flow of air to the distillation column iskept constant; however, the air flow sent to the turbine is reduced,which means that the air flow sent to the cold box is also reducedduring the repressurization cycle. This also results in differingconditions within the heat exchanger located in the cold box.

U.S. Pat. No. 6,073,463 adjusts rich liquid and waste flows to maintainproduct purities during repressurization since the air flow sent to thecold box is not kept constant.

Similarly, WO 2007/033838 adjusts the product flow from the cold box tocompensate for the loss of air sent to the cold box duringpressurization. A product gas buffer is added in the product stream inorder to provide constant product flow to the customer. However, each ofthe above mentioned solutions suffer additional problems.

Therefore, there is a need for a process in which the MAC can operate ata substantially constant capacity (i.e., the MAC capacity is notpurposefully adjusted during the repressurization step) while alsomaintaining a substantially constant flow of air to the cold box (i.e.,the flow rate of air to the cold box is not purposefully adjusted duringthe repressurization step) such that process conditions within the coldbox (e.g., flows in and out, temperatures, etc . . . ), and not just thedistillation column, remain unchanged.

Therefore, it would be desirable to have an improved apparatus andmethod that avoids these added expenses and operates in an overall moreefficient manner.

SUMMARY OF THE INVENTION

The present invention is directed to a method and apparatus thatsatisfies at least one of these needs. In one embodiment, the method caninclude using an available compressed dry gas such as nitrogen, oxygen,stored purified air, or synthetic air to repressurize the adsorberwithout diverting any of the purified air just exiting the currentlyon-line adsorber or changing the flow rate of the main air compressor orair sent to the cold box. This enables the main air compressor (MAC) tooperate at a relatively constant flow rate while also sending arelatively constant air flow to the cold box during thisrepressurization step, thereby reducing the risks of process upsets andminimizing capital expenditures related to the MAC and other warm-endequipments. In one embodiment, the dry gas can be sourced from anexternal source.

In one embodiment, the dry gas can be nitrogen. In one embodiment, thenitrogen can originate from external sources such as from a nearbypipeline or can be a pressurized nitrogen stream coming from the coldbox (either directly from the cold box or after a compressor if it is ata lower pressure (e.g., waste nitrogen stream or low pressure nitrogenproduct). In another embodiment, the dry gas can be purified air. In oneembodiment, the purified air can originate from a high pressure airbuffer tank.

In one embodiment in which nitrogen is used as the dry gas, it ispreferable to extend the time for the vessel changeover step (e.g., bothfirst adsorber A and second adsorber B receive wet air 2 and operate ina parallel adsorption cycle) in order to mitigate any process upset onthe air separation unit (ASU) due to the change in gas compositionduring this changeover. In one embodiment, this is accomplished by useof small nitrogen blending valves. In another embodiment, an automaticincrease in column liquid nitrogen reflux flow and liquid nitrogenwithdraw can be implemented to further minimize this disturbance.

In one embodiment, a method for the production of air gases by thecryogenic separation of air with front end purification and aircompression can include the steps of:

-   -   a) compressing atmospheric air to a pressure suitable for the        cryogenic rectification of air to produce a compressed wet air        stream;    -   b) purifying a compressed wet air stream of water and carbon        dioxide within a front end purification system to produce a dry        air stream having reduced amounts of water and carbon dioxide as        compared to the compressed wet air stream, wherein the front end        purification system comprises a first vessel and a second vessel        configured in a permutable fashion, wherein the first vessel        comprises a first adsorber and the second vessel comprises a        second adsorber, wherein the first and second adsorbers operate        in alternating cycles such that while the first adsorber is in        an adsorption cycle, the second adsorber is in a regeneration        cycle and while the second adsorber is in the adsorption cycle,        the first adsorber is in the regeneration cycle;    -   c) introducing the dry air stream to a cold box under conditions        effective to separate air into a nitrogen enriched stream and an        oxygen enriched stream; and    -   d) withdrawing the nitrogen enriched stream and the oxygen        enriched stream from the cold box.

In one embodiment, the regeneration cycle for each vessel of the frontend purification system can further includes the steps of: 1)depressurizing the vessel from an adsorption pressure to a regenerationpressure that is sufficiently low to release water and carbon dioxidefrom a surface of an adsorbent material within the vessel; 2)regenerating the adsorbent material using a first dry gas; and 3)pressurizing the vessel to the adsorption pressure using a second gas.In one embodiment, the second gas used in step 3) of the regenerationcycle is a second dry gas not sourced directly from the vessel that isin the adsorption cycle. In one embodiment, the flow rate of the dry airstream introduced to the cold box remains substantially constant duringstep c) regardless of adsorption cycle.

In optional embodiments of the method for the production of air gases bythe cryogenic separation of air with front end purification and aircompression:

-   -   step 2) of the regeneration cycle for each vessel of the front        end purification system can further include the steps of 2a)        heating the adsorbent material to a regeneration temperature TR        by heating the dry gas upstream the vessel for a first period of        time and then 2b) cooling the adsorbent material to a second        temperature T₂ by continuing to introduce the dry gas into the        vessel, but without adding heat to the dry gas upstream of the        vessel;    -   the flow rate of the compressed wet air stream sent to the front        end purification system remains substantially constant during        steps b) and 3);    -   the method includes an absence of the steps of: increasing the        flow rate of the compressed wet air stream sent to the front end        purification system; and decreasing the flow rate of the dry air        stream introduced to the cold box (e.g., the flow rate of        compressed air from the MAC stays relatively constant throughout        the entire cycle while also keeping the flow of dry air to the        cold box relatively constant throughout the entire cycle as        well);    -   step 3) of the regeneration cycle includes an absence of sending        a portion of the dry air stream from the first vessel to the        second vessel when the second vessel is pressurizing;    -   the first dry gas comprises the nitrogen enriched stream from        the cold box;    -   the second dry gas comprises the nitrogen enriched stream from        the cold box;    -   the second dry gas comprises a dry gas stream from an external        source;    -   the second dry gas is a synthetic air stream having a        composition similar to that of air, wherein the synthetic air        stream consists essentially of oxygen and nitrogen sourced from        the cold box;    -   the second dry gas comprises nitrogen and oxygen, wherein the        nitrogen content is between 70 and 88% and the oxygen content is        between 12 and 30%;    -   the second dry gas is sourced from a compressed air storage        tank, wherein the compressed air storage tank is in fluid        communication with the front end purification system, such that        the compressed air storage tank is configured to receive a        portion of the dry air stream exiting the front end purification        system prior to the dry air stream being introduced to the cold        box;    -   the method further comprises a switch over step following        step 3) of the regeneration cycle in which both the first        adsorber and the second adsorber are adsorbing in a parallel        fashion;    -   during the course of the switch over step, flow of the        compressed wet air stream is gradually increased to the adsorber        that just finished its pressurizing step;    -   the rate of increasing the flow of the compressed wet air stream        to the adsorber that just finished its pressurizing step is        adjusted based on the composition of the dry gas sent to the        cold box or the composition of the dry gas exiting one or more        of the vessels or the composition of gas being used to        pressurize the vessel;    -   the method further includes a step of monitoring the composition        of the purified gas at a location selected from within the front        end purification system or between the front end purification        system and the cold box.

In another aspect of the invention, an apparatus for the production ofair gases by the cryogenic separation of air with front end purificationand air compression is provided. In one embodiment, the apparatus mayinclude: a main air compressor configured to compress atmospheric air toa pressure suitable for the cryogenic rectification of air to produce acompressed wet air stream; a front end purification system in fluidcommunication with the main air compressor, such that the front endpurification system is configured to receive the compressed wet airstream from the main air compressor and purify the compressed wet airstream of water and carbon dioxide to produce a dry air stream havingreduced amounts of water and carbon dioxide as compared to thecompressed wet air stream, wherein the front end purification systemcomprises a first vessel and a second vessel configured in a permutablefashion, wherein the first vessel comprises a first adsorber and thesecond vessel comprises a second adsorber, wherein the first and secondadsorbers operate in alternating cycles such that while the firstadsorber is in an adsorption cycle, the second adsorber is in aregeneration cycle and while the second adsorber is in the adsorptioncycle, the first adsorber is in the regeneration cycle; and a cold boxin fluid communication with the front end purification system, such thatthe cold box is configured to receive the dry air stream from the frontend purification system and separate the dry air stream into a nitrogenenriched stream and an oxygen enriched stream.

In one embodiment, the regeneration cycle of each vessel of the frontend purification system is configured to:

-   -   1) depressurize the vessel from an adsorption pressure to a        regeneration pressure that is sufficiently low to release water        and carbon dioxide from a surface of an adsorbent material        within the vessel;    -   2) regenerate the adsorbent material using a first dry gas; and    -   3) pressurize the vessel to the adsorption pressure using a        second dry gas.

In one embodiment, the apparatus has an absence of a flow meansconfigured to transfer a portion of the dry air stream from the firstadsorber directly to the second adsorber when the second adsorber is inits pressurization cycle.

In another embodiment, the main air compressor is configured to operateat substantially the same flow rate during all cycles of the front endpurification system, and wherein the cold box is configured to operateat substantially the same flow rate of the dry air stream dry air duringall cycles of the front end purification system.

In another embodiment, the apparatus may also include a compressed airstorage tank configured to source the second dry gas, wherein thecompressed air storage tank is in fluid communication with the front endpurification system, such that the compressed air storage tank isconfigured to receive a portion of the dry air stream exiting the frontend purification system prior to the dry air stream being introduced tothe cold box. In one embodiment, the compressed air storage tank isconfigured to receive a continuous flow of the portion of the dry airstream. In one embodiment, the compressed air storage tank is configuredto provide the second dry gas to the front end purification systemduring repressurization intermittently. In one embodiment, the flow rateof the portion of the dry air stream sent to the air storage tank isless than 1% of the total flow of air from the MAC.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, claims, and accompanying drawings. It is to be noted,however, that the drawings illustrate only several embodiments of theinvention and are therefore not to be considered limiting of theinvention's scope as it can admit to other equally effectiveembodiments.

FIG. 1 provides an embodiment of the prior art in which vessel A isadsorbing and vessel B is being regenerated.

FIG. 2 provides an embodiment of the prior art in which vessel A isadsorbing and vessel B is being pressurized.

FIG. 3 provides an embodiment of the present invention in which vessel Ais adsorbing and vessel B is being regenerated.

FIG. 4 provides an embodiment of the present invention in which vessel Ais adsorbing and vessel B is being pressurized.

FIG. 5 provides an embodiment of the present invention in which bothvessels are adsorbing.

FIG. 6 provides a graphical comparison between embodiments of thecurrent invention and that of the prior art.

FIG. 7 provides a graphical representation of the main air compressorflow rate for an embodiment of the prior art.

FIG. 8 provides a graphical representation of the main air compressorflow rate for an embodiment of the present invention.

FIG. 9 provides a graphical representation of the flow rate of dry airsent to the cold box for an embodiment of the prior art.

FIG. 10 provides a graphical representation of the flow rate of dry airsent to the cold box for an embodiment of the present invention.

DETAILED DESCRIPTION

While the invention will be described in connection with severalembodiments, it will be understood that it is not intended to limit theinvention to those embodiments. On the contrary, it is intended to coverall the alternatives, modifications and equivalence as may be includedwithin the spirit and scope of the invention defined by the appendedclaims.

FIGS. 3 and 4 represent embodiments of the present invention. As in FIG.1, FIG. 3 represents a flow diagram of front end purification unit 10,in which first adsorber A is in its adsorption cycle and second adsorberB is in the cleaning stage of its regeneration cycle. In this setup,valves 3A, 4A, 5B and 6B are open, while valves 3B, 4B, 5A, 6A, 7, 8A,8B, 9A, and 9B are closed. As shown in FIG. 3, the flow of gases (airand regeneration gas) during adsorption and cleaning stage ofregeneration are largely unchanged. The primary difference between thesetups of FIG. 1 and FIG. 3 is the presence of pressurizing gas line 42and valves 8A and 8B. Bypass valves 9A and 9B are also included, butthey are optional, as will be explained in more detail. In a preferredembodiment, pressurizing gas line 42 contains a dry pressurizing gas.However, a wet pressurizing gas can also be used.

Another optional element is purified air slip stream 28 and compressedair storage 30. In this optional embodiment, a small portion of purifiedair can be compressed 29 sent to the compressed air storage 30, suchthat the process can have a storage tank of air, which can be useful forvarious purposes. In embodiments including compressed air storage 30,not all of the air compressed by the MAC and purified by the adsorberswill be sent to the cold box 20. In one embodiment, purified air slipstream 28 can be a continuous flow, preferably at a constant flow rate.In one embodiment, the percentage flow of purified air slip stream 28 ascompared to the flow rate of compressed wet air 2 can be based on theamount of dry gas used during the repressurization vis-à-vis the totaladsorber cycle (i.e., adsorption, depressurization, regeneration), andtherefore, can be of the order of less than 2% of the flow rate ofcompressed wet air 2, preferably less than 1%, preferably less than0.5%, more preferably about 0.05-0.5%.

Similarly to FIG. 2, FIG. 4 represents a flow diagram of front endpurification unit 10 in which first adsorber A is still in an adsorptioncycle and second adsorber B is in a repressurization phase (pressurizingback to normal adsorption pressure (e.g., high pressure) to get readyfor its adsorption phase). However, instead of using purified air comingfrom first adsorber A to pressurize second adsorber B by opening valve7, second adsorber B can be pressurized by introducing a dry,pressurized gas from an external source 40. It is preferable that thegas from the external source 40 is dry; however, in certain embodimentsthe pressurized gas can be wet.

In the embodiment shown, 5 moles of pressurizing gas via line 42 passthrough valve 8B and into second adsorber B. This allows the flow ratecoming from the MAC to remain unchanged (100 moles in this example)while also keeping the flow rate of purified, dry gas 12 sent to thecold box 20 to also remain constant.

In one embodiment, valves 3A, 3B, 4A, 4B, 5A, 5B, 6A, and 6B, are on/offvalves, meaning that they are configured to either be opened or closed,and do not partially open. Valves 7, 8A, 8B, 9A, and 9B are preferablyconfigured to control the flow rates through their respective lines andcan be set to any point between closed and fully open.

In one embodiment, the dry, pressurizing gas 42 can be a pressurizednitrogen rich product stream from the cold box. In another embodiment,the pressurizing gas 42 can come from the same source as theregeneration gas 14, such as a waste nitrogen stream coming from thecold box (e.g., stream 44 by passing through valves 21 and 25 and thenpressurized to an appropriate pressure via a pressurizing means notshown). Other sources for pressurizing gas 42 can include nearbypipelines (e.g., a pressurized nitrogen pipeline), pressurized gasvaporized from a liquid storage tank (e.g., nitrogen and/or oxygenstorage tank). In yet another embodiment, the pressurizing gas 42 can beintroduced to the top of the adsorbers A, B by reversing flow throughline 44 and going through valves 5A, 5B as appropriate.

In an alternate embodiment, pressurizing gas 42 can be a synthetic airstream made of nitrogen 22 (e.g., product and/or waste nitrogen) andoxygen 26 coming from the cold box 20 in respective amounts thatsimulate the composition of air. Valves 21 and 23 can be configured tocontrol the flow rates of the two gasses appropriately. This preferablyallows the pressurization gas to have the same composition as that ofair without using any of the purified air coming out of the adsorbervessels A or B. Alternatively, in embodiments with compressed airstorage 30, compressed air 32 can be used as the pressurizing gas 42.

In one embodiment, front end purification unit can include bypass valves9A and 9B. In embodiments in which the pressurizing gas 42 has acomposition different from air, the process can include a switch cyclein which both first adsorber A and second adsorber B receive wet air 2and operate in a parallel adsorption cycle. This allows for thecomposition within second adsorber B to slowly equilibrate to that ofair while also maintaining the composition of the dry air stream sent tothe cold box, thereby reducing any potential process upsets associatedwith sending a significantly different composition to the cold box.

As shown in FIG. 5, valves 3A, 4A, 4B and 9B are set to open, whilevalves 3B, 5A, 5B, 6A, 6B, 7, 8A, 8B, 7, 9A are set to closed. In oneembodiment, valve 9B is only slightly open and then gradually opens moreand more until the composition within second adsorber B is that of air.In one embodiment, the process can include a step of measuring thecomposition of the purified gas exiting one or both of the adsorbers,and once they match or are within the tolerance range (e.g.,approximately 78% nitrogen), valve 3B can be set to fully open beforeclosing valve 9B, while also closing valves 3A and 4A, thereby placingsecond adsorber B in the adsorption cycle. In another embodiment, theopening and closing of valves 3A, 3B, 4A and 9B can be set to apredetermined time delay. In one embodiment, this time delay can bebased on the time needed to allow adsorber B to approach a compositionsimilar to that of air. Following the switchover, an appropriatedepressurizing valve (not shown) would be opened to despressurizeadsorber A to essentially atmospheric pressure and then valves 5A and 6Awould be opened and the depressurizing valve (not shown) would be closedin order to start the regeneration cycle of first adsorber A.

Alternatively at the beginning of the parallel run, valve 3A can beclosed while valve 9A can be set to fully open. Valve 9A can thensubsequently be adjusted in step with the gradual adjustment of valve 9Bin order to provide additional fine tuning.

In one embodiment, gas analyzers 50, 55 can be in electric communicationwith a controller (not shown) that is configured to adjust the biases ofvalves, for example valves 3A, 3B, 9A and 9B, based on the compositionof the purified dry gas exiting the first and second adsorbers. Those ofordinary skill in the art will also recognize that flow indicators couldbe included in order to follow the flow rates of various streams.However, these flow indicators have been left off in order to providemore clarity for the figures.

For illustrative purposes, in one embodiment, the adsorption pressurecan be at least 4 bar, preferably between 5-6 bar. Regeneration pressureis preferably just above atmospheric pressure; however, those ofordinary skill in the art will recognize that the pressure can beanything lower than the adsorption pressure that is still effective forremoving the adsorbed impurities. In another embodiment, adsorptiontemperature can be around 55° F.±15° F., while the heating cycle of theregeneration phase can be approximately 300° F. Again, those of ordinaryskill in the art will recognize that the temperature can be adjusted inorder to improve adsorption or regeneration conditions. In a preferredembodiment, the adsorption cycle can last between 1.5 to 4 hours, withthe regeneration cycle also being between 1.5 to 4 hours. In a preferredembodiment, the regeneration cycle can include a depressurization steplasting less than 10 minutes, a heating and subsequent cooling step, apressurization step of about 10 minutes, and then a switching step ofabout 10 minutes. In one embodiment, the heating step can last about 36to 84 minutes and the cooling step can last about 54 to 126 minutes. Ina preferred embodiment, the cooling step is approximately 50% longerthan the heating step.

FIG. 6 provides a comparison view of the theoretical advantage ofembodiments of the present invention as compared to the methods of theprior art. The top line shows that embodiments of the current inventioncan flow at approximately 100% of the available MAC flow continuously.The bottom portion of the graph shows that the methods of the prior arthave to run at approximately 95% of the available MAC capacity and haveperiods of 100% available MAC flow only during the pressurization phase.

WORKING EXAMPLES

FIGS. 7-10 provide actual data for setups of the prior art as comparedto an embodiment of the present invention. FIG. 7 provides a graphicalrepresentation of MAC air flow rate versus time for methods of the priorart. As can be seen, during periods of pressurization, the MAC air flowrate drastically rises and falls during this period. FIG. 8 provides thesame graphical representation of MAC air flow rate as a function of timefor an embodiment of the present invention. As can be clearly seen, theMAC air flow rate is much smoother and does not have drastic spikes. Notonly does this allow the MAC to be sized in a more efficient manner, itcan also help to extend the life of the MAC as it is not experiencingdrastic fluctuations throughout its lifecycle.

FIGS. 9 and 10 provide graphical representations of the air flow sentfrom the front end purification system to the cold box for the methodsof the prior art and methods in accordance with an embodiment of thepresent invention, respectively. While not as pronounced as the MAC airflow rate, embodiments of the present invention advantageously provide asmaller standard deviation to the cold box (5.72 vs. 4.60). This can beattributed to less disruptions during the process caused by the spikingof the MAC.

The terms “nitrogen-rich” and “oxygen-rich” will be understood by thoseskilled in the art to be in reference to the composition of air. Assuch, nitrogen-rich encompasses a fluid having a nitrogen contentgreater than that of air. Similarly, oxygen-rich encompasses a fluidhaving an oxygen content greater than that of air. The term “dry” as itpertains to gases will be understood by those skilled in the art toencompass a gas that has reduced amounts of water vapor as compared tothe contaminated wet air (i.e., local atmospheric air).

While the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. Accordingly, it is intendedto embrace all such alternatives, modifications, and variations as fallwithin the spirit and broad scope of the appended claims. The presentinvention may suitably comprise, consist or consist essentially of theelements disclosed and may be practiced in the absence of an element notdisclosed. Furthermore, if there is language referring to order, such asfirst and second, it should be understood in an exemplary sense and notin a limiting sense. For example, it can be recognized by those skilledin the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unlessthe context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means thesubsequently identified claim elements are a nonexclusive listing (i.e.,anything else may be additionally included and remain within the scopeof “comprising”). “Comprising” as used herein may be replaced by themore limited transitional terms “consisting essentially of” and“consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, makingavailable, or preparing something. The step may be performed by anyactor in the absence of express language in the claim to the contrary.

Optional or optionally means that the subsequently described event orcircumstances may or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such a range isexpressed, it is to be understood that another embodiment is from theone particular value and/or to the other particular value, along withall combinations within said range.

All references identified herein are each hereby incorporated byreference into this application in their entireties, as well as for thespecific information for which each is cited.

We claim:
 1. A method for reducing process disturbances duringpressurization of an adsorber in an air separation unit, wherein the airseparation unit comprises a front end purification unit, a main aircompressor, a cold box having a main heat exchanger and a distillationcolumn system disposed therein, and an air buffer tank, wherein thefront end purification unit comprises a first adsorber and a secondadsorber, the method comprising the steps of: pressurizing the firstadsorber while the second adsorber operates in an adsorption cycle,wherein the step of pressurizing the first adsorber further comprisesthe steps of withdrawing a pressurized air stream from the air buffertank and introducing the pressurized air stream to the first adsorberuntil the first adsorber is at a target pressure, wherein the air buffertank is in fluid communication with an outlet of the front endpurification unit, wherein the method further comprises the step ofcontinually sending a first portion of purified air flow from the frontend purification unit to the air buffer tank.
 2. The method as claimedin claim 1, wherein the first portion of purified air flow from thefront end purification unit is between 0.3% to 1% of total air flowinginto the booster air compressor.
 3. The method as claimed in claim 1,wherein the first portion of purified air flow is first compressed in abooster air compressor prior to being sent to the air buffer tank.
 4. Amethod for reducing process disturbances during pressurization of anadsorber in an air separation unit, wherein the air separation unitcomprises a front end purification unit, a main air compressor, a coldbox having a main heat exchanger and a distillation column systemdisposed therein, a booster air compressor, and an air buffer tank,wherein the front end purification unit comprises a first adsorber and asecond adsorber, the method comprising the steps of: compressing an airstream in a main air compressor to form a compressed main air stream;purifying the compressed main air stream in the front end purificationunit to remove water and carbon dioxide to form a dry main air stream;sending a first portion of the dry main air stream to the cold box forcooling and rectification therein; boosting a second portion of the drymain air stream to a higher pressure P_(H) in the booster air compressorto produce a boosted air stream; and sending the boosted air stream tothe air buffer tank, wherein each of the adsorbers of the front endpurification unit undergoes a processing cycle comprising a regenerationcycle, a pressurization cycle, and an adsorption cycle, wherein duringthe pressurization cycle, a pressurized air stream is withdrawn from theair buffer tank and introduced to the adsorber that is undergoingpressurization.
 5. The method as claimed in claim 4, wherein thepressurized air stream is only withdrawn from the air buffer tank andintroduced to the adsorber during the pressurization cycle.
 6. Themethod as claimed in claim 4, wherein the flow rate of the boosted airstream sent to the air buffer tank is between 0.3% to 1% of the flowrate of the dry main air stream.
 7. The method as claimed in claim 4,wherein the boosted air stream is sent to the air buffer tank at aconstant rate during the entire processing cycle of the front endpurification unit.
 8. An apparatus for reducing process disturbancesduring pressurization of an adsorber in an air separation unit, whereinthe apparatus comprises: a main air compressor configured to compress anair stream to form a compressed main air stream; a front endpurification unit configured to purify the compressed main air stream ofwater and carbon dioxide to form a dry main air stream, wherein thefront end purification unit comprises two adsorbers, wherein eachadsorber is configured to operate with an adsorption cycle, aregeneration cycle, and a pressurization cycle; a booster air compressorin fluid communication with the front end purification unit, wherein thebooster air compressor is configured to boost a second portion of thedry main air stream to a higher pressure PH to form a boosted airstream; an air buffer tank having an air inlet in fluid communicationwith an outlet of the booster air compressor, wherein the air buffertank is configured to receive the boosted air stream, wherein an outletof the air buffer tank is in fluid communication with the front endpurification unit; a cold box in fluid communication with the front endpurification unit, wherein the cold box is configured to receive a firstportion of the dry main air stream, wherein the cold box houses a mainheat exchanger and a distillation column system, wherein the main heatexchanger is configured to cool the first portion of the dry main airstream to a cryogenic temperature suitable for rectification of air,wherein the distillation column system is configured to receive thefirst portion of the dry main air stream from the main heat exchangerafter cooling, wherein the distillation column system is configured toseparate the dry main air stream into nitrogen and oxygen; whereinduring a pressurization cycle of each adsorber, a valve located betweenthe air buffer tank and the front end purification unit is configured toopen to allow for dry air to flow from the air buffer tank to theadsorber, wherein the valve is configured to close, thereby stopping theflow of dry air to the adsorber, once the pressurization cycle iscompleted.
 9. The apparatus as claimed in claim 8, further comprisingmeans for regulating the flow rate of the boosted air stream sent to theair buffer tank.
 10. The apparatus as claimed in claim 9, wherein theflow rate of the boosted air stream sent to the air buffer tank isbetween 0.3% to 1% of the flow rate of the dry main air stream.
 11. Anapparatus for the production of air gases by the cryogenic separation ofair with front end purification and air compression, the apparatuscomprising: a main air compressor configured to compress atmospheric airto a pressure suitable for the cryogenic rectification of air to producea compressed wet air stream; a front end purification system in fluidcommunication with the main air compressor, such that the front endpurification system is configured to receive the compressed wet airstream from the main air compressor and purify the compressed wet airstream of water and carbon dioxide to produce a dry air stream havingreduced amounts of water and carbon dioxide as compared to thecompressed wet air stream, wherein the front end purification systemcomprises a first vessel and a second vessel configured in a permutablefashion, wherein the first vessel comprises a first adsorber and thesecond vessel comprises a second adsorber, wherein the first and secondadsorbers operate in alternating cycles such that while the firstadsorber is in an adsorption cycle, the second adsorber is in aregeneration cycle and while the second adsorber is in the adsorptioncycle, the first adsorber is in the regeneration cycle; a cold box influid communication with the front end purification system, such thatthe cold box is configured to receive the dry air stream from the frontend purification system and separate the dry air stream into a nitrogenenriched stream and an oxygen enriched stream, wherein the regenerationcycle of each vessel of the front end purification system is configuredto: 1) depressurize the vessel from an adsorption pressure to aregeneration pressure that is sufficiently low to release water andcarbon dioxide from a surface of an adsorbent material within thevessel; 2) regenerate the adsorbent material using a first dry gas; and3) pressurize the vessel to the adsorption pressure using a second gas;and an absence of a flow means configured to transfer a portion of thedry air stream from the first adsorber directly to the second adsorberwhen the second adsorber is in its pressurization cycle.
 12. Theapparatus as claimed in claim 11, wherein the main air compressor isconfigured to operate at substantially the same flow rate during allcycles of the front end purification system, and wherein the cold box isconfigured to operate at substantially the same flow rate of the dry airstream dry air during all cycles of the front end purification system.13. The apparatus as claimed in claim 11, further comprising acompressed air storage tank configured to source the second gas, whereinthe compressed air storage tank is in fluid communication with the frontend purification system, such that the compressed air storage tank isconfigured to receive a portion of the dry air stream exiting the frontend purification system prior to the dry air stream being introduced tothe cold box.